The T3 suppression test by the 24-hr thyroidal 131I uptake was reevaluated in patients with Graves' disease before and after withdrawal of antithyroid drug. Fifty patients had been treated with propylthiouracil (PTU) or methylmercaptoimidazole (MMI) for 12 to 70 months. They were prescribed a maintenance dose of antithyroid drug (PTU, 50 mg/day; MMI, 5 mg/day) at the time of investigation and regarded as euthyroid on the basis of serum T3, T4 and TSH levels. Each patient was given 75 micrograms T3 daily for 8 days in conjunction with PTU or MMI. The 24-hr thyroidal 131I uptake was then measured (post T3 uptake). In 30 patients whose post T3 uptake was below 35%, treatment was stopped and the T3 suppression test was repeated at one and 3 months later. During the two-year follow up, 24 remained well, while 6 relapsed within 4 to 12 months. In patients with sustained remission, the post T3 uptake was significantly lower in the MMI-treated group (13 cases, 7.7 +/- 1.0%) than in the PTU-treated group (11 cases, 18.6 +/- 1.9%). MMI withdrawal produced a marked rebound in the post T3 uptake, whereas none of the patients showed the rebound after PTU withdrawal. In patients who relapsed later, there was no difference in the post T3 uptake during treatment and the rebound occurred in the both groups following goitrogen withdrawal. Serum T3, T4 and TSH levels were within normal ranges at one and 3 months after cessation of antithyroid drug. From the results of the present study, it is concluded that criteria for T3 suppressibility by the 24-hr uptake should be determined by the antithyroid drug employed and by the time of investigation. There is a dissociation in the post T3 uptake values following withdrawal of the two different antithyroid drugs.